For synthesis of UF6 in uranium enrichment in energy industries, fluorine gas has been used on a mass scale for a long period of time.
Also, fluorine gas is essential for synthesis of industrially useful functional materials such as water- and oil-repelling agent, active material for lithium battery, a dry etchant for production of semiconductors, a fluoro polymer for production of semiconductors, an additive for polymer materials and an intermediate for pharmaceuticals, and an amount of fluorine gas used therefor has been increasing year by year.
Further, it is strongly expected that use of fluorine gas as a gas for dry etching or cleaning for production of next generation semiconductor and liquid crystal, and as a gas for CVD will expand.
However, fluorine gas has extremely high reactivity and corrosiveness, and very high technical capability is required for storing and handling thereof. Therefore, significant restriction cannot help being placed on the use of fluorine gas.
Namely, in the case of storing fluorine gas in a metallic cylinder, for the purpose of securing safety, not only its pressure needs to be decreased to 2 Mpa or less, but also it cannot help being diluted previously with nitrogen or the like before being filled in the cylinder. In addition, for taking fluorine gas out of the cylinder, it is necessary to provide so many safety measures such as a valve device, a pressure reducing device and a safety device, and from this point of view, economical efficiency and productivity are lacking for the use of fluorine gas. Further there is a problem that even in the case of high purity fluorine gas subjected to sufficient refining before filling in a cylinder, it is subject to contamination due to corrosive products (for example, various metal fluorides) attributable to materials of a cylinder and a valve device, and especially in the case of application to production of semiconductors, it is necessary to take measures, for example, to separately provide refining equipment.
On the other hand, a method of directly using fluorine gas generated by electrolysis of a molten salt containing hydrogen fluoride has been employed. However, in this method, it is necessary to make thoroughgoing preparation for sufficient safety measures such as enough space for maintenance of safety and electrolyzer room provided with complete shielding, and in addition, it is necessary to secure a large-scale rectifier, refining equipment and emissions cleaning equipment and arrange operation and maintenance personnel having high technical capability at necessary places. Further, high purity fluorine gas cannot be taken out immediately after turning on electrolysis equipment, and it is necessary to carry out preliminary electrolyzing for a long period of time. In addition, there is a problem that when electrolyzing is continued for a long period of time, an anode effect occurs suddenly and frequently electrolyzing is obliged to be interrupted. Therefore, this method lacks economical efficiency and productivity.
Also, there is known a method of using a metal fluoride as a fluorine storage material and desorbing fluorine gas by thermal decomposition of the metal fluoride. For example, K3NiF6 is fluorinated to prepare K3NiF7, and by subjecting K3NiF7 to thermal decomposition, K3NiF6 is obtained and fluorine gas can be generated (trade name F-GENE, fluorine generator available from SHOWA DENKO K.K.). However, in this method, there is a problem that a fluorine storage amount per unit mass of K3NiF7 is theoretically as small as 7.0% by mass.
On the other hand, there is proposed a method of using a carbon fiber as a fluorine storage material (Ching-chen Hung, Donald Kucera, Industrial applications of graphite fluoride fibers, NASA-CP-3109-VOL-1, pp. 156-164 (1991)). Carbon fibers are lighter than metal fluorides, and even if a fluorine storage amount per unit mass of fluorinated carbon fiber is 50.7% by mass, an amount of gas to be effectively desorbed is at most 22% by mass including impurity gas. Also, there is a problem that not less than 10% by mass of fluorocarbon gases such as CF4 and C2F6 are generated as impurities in the generated gases. In addition, from the view point of practical use, there is a fatal problem that carbon fibers are physically broken as fluorine storing and releasing cycles proceed, and cannot be used repeatedly.
JP2005-273070A proposes a method of fluorinating a carbon nanotube and heating the obtained fluorinated carbon nanotube to desorb fluorine gas. According to this method, while a fluorine storage amount per unit mass is increased, in the case of a fluorination reaction temperature of 200° C., a fluorine storage amount per unit mass of fluorinated carbon nanotube is at most about 52.9% by mass. Fluorine is desorbed only by heating, and there is no option of selecting a method of desorbing fluorine gas. Even in this method, there are problems that significant amounts of fluorocarbon gases such as CF4 and C2F6 are generated as impurities and carbon nanotubes are physically broken as a result of repeated fluorine storing and releasing cycles, and those problems have not yet been solved.
In recent years, as a result of a rise of nano technology, a material called carbon nanohorn has been developed, and changes in structure and physical properties resulting from its fluorination have been studied as disclosed in detail in a bulletin of The New Industry Research Organization, Vol. 25, No. 3 (Serial Number 99), Sept. 2005, pp. 6-11 and Journal of Physical Chemistry B, 108 (28), 9614-9618 (2004). There are descriptions with respect to high density storing of hydrogen gas and methane gas at normal temperature, and it is indicated that chemical bond will be able to be formed by surface modification of the carbon nanohorn with fluorine and that effective molecular adsorption site will be able to be formed by controlling electronic state of the carbon nanohorn. However, there are no description and teaching as to storing of fluorine. This is because an adsorption theory of hydrogen gas and methane gas is substantially different from that of fluorine gas.
Namely, hydrogen gas and methane gas are trapped in a specific molecular potential site formed in the carbon nanohorn, and do not form chemical bond with the carbon nanohorn, in other words, it is merely a so-called physical adsorption. On the contrary, fluorine gas forms covalent bond or semi-ionic bond with carbon atoms constituting the carbon nanohorn, namely a so-called chemical adsorption. The both are definitely different from each other in the mentioned point and cannot be discussed on the same level.
Further, on the way of fully studying electrochemical properties of fluorinated carbon nanohorn, as disclosed in a manuscript of the 32nd meeting of The Carbon Society of Japan, Dec. 7, 2005, pp. 132-133, it has been made clear that in the case of using carbon nanohorn as an active material for positive electrode of lithium battery, there are characteristics that discharging reaction proceeds by uniform solid phase reaction, an initial electromotive force is as high as 4.2 V, and not only energy density is high, but also an electromotive force is gradually decreased as discharging proceeds. This indicates superiority in practical use on batteries in that not only a service life of lithium battery is made longer, but also remaining battery capacity can be always monitored, thereby enabling sudden running down of the battery to be avoided. However, there is no teaching at all with respect to storing and releasing of fluorine gas.